fix(test): resolve flakiness in MaxIdleConnections tests

[olavloite]

Resolves the flakiness in TestMaxIdleConnectionsNonZero and TestMaxIdleConnectionsZero by running them with multiplexed sessions disabled, configuring minSessions=1, and counting the created sessions. Drains requests after shutting down the client/server during teardown. Additionally, fixes an infinite loop bug in DrainRequestsFromServer on closed channel, and removes an unused background client in setupTestDBConnectionWithParamsAndDialect.

Fixes #816

[gemini-code-assist]

Code Review

This pull request refactors several Spanner driver tests to handle multiplexed sessions correctly, simplifies connection closing, and introduces a helper function countCreatedSessions to count session creation requests. It also updates DrainRequestsFromServer to safely handle closed channels. The review feedback suggests two improvements: storing the channel in a local variable before the loop in DrainRequestsFromServer to avoid repeated method calls, and using a type switch in countCreatedSessions for more idiomatic Go code.

[olavloite]

Excellent fix for MaxIdleConnections flakiness. Disabling multiplexed sessions and checking session creation requests directly makes the tests deterministic. Also, fixing the infinite loop in DrainRequestsFromServer when the channel is closed is a crucial bug fix for the test utilities. Clean work.